modules/virtualisation/qemu-vm: always enable serial console

Always enable both tty and serial console, but set preferred console
depending on cfg.graphical.
Even in qemu graphical mode, you can switch to the serial console via
Ctrl+Alt+3.

With that being done, you also don't need to specify
`systemd.services."serial-getty@ttyS0".enable = true;` either as described in
https://nixos.wiki/wiki/Cheatsheet#Building_a_service_as_a_VM_.28for_testing.29,
as systemd automatically spawns a getty on consoles passwd via cmdline.

This also means, vms built by 'nixos-rebuild build-vm' can simply be run
properly in nographic mode by appending `-nographic` to `result/bin/run-*-vm`,
without the need to explicitly add platform-specific QEMU_KERNEL_PARAMS.